A bill to legalise assisted suicide in Scotland is expected to be brought before the Scottish parliament this autumn after the independent MSP Margo MacDonald won cross-party backing.

People with a progressive and irreversible illness, the terminally ill, or those who had an "intolerable" quality of life, could get a doctor's help to kill themselves under the proposed law.

MacDonald said today that these measures would help people such as Debbie Purdy, the multiple sclerosis suffer whose case is being decided by the House of Lords today, and could have helped the rugby player Dan James, who killed himself at the Dignitas clinic in Switzerland last year after he was paralysed in a scrum.

In a significant indication of the changing political climate on assisted suicide, MacDonald's private member's bill has been formally supported by 21 MSPs from most parties at Holyrood, which means the proposals will be investigated by parliamentary committees and then debated by the full parliament.

Private bills at Holyrood require at least 18 signatories before they can be presented.

The last attempt to table an assisted suicide bill at Holyrood, by the Liberal Democrat MSP Jeremy Purvis in 2005, failed because he secured only 12 MSPs' signatures in the face of a concerted campaign against the measure from medical and religious organisations.

MacDonald, 66, said a vote by the Royal College of Nursing on Saturday to take a neutral stance on the issue, the rise in the number of Britons going to Switzerland to kill themselves and repeated opinion polls showing majority support on the issue had significantly influenced MSPs.

"All the evidence that has come to light over the last few months has strengthened my belief I was right to raise the matter and join the debate, and I'm right to raise a bill and test public opinion," she said.

MacDonald, previously a Scottish National party MP for Glasgow Govan, has Parkinson's disease. In an emotional speech at Holyrood in 2008, she said she wanted the right to die and to ensure that if she did eventually choose assisted suicide, her husband, Jim Sillars, also a former SNP MP, could not be prosecuted.

Unlike in England, the law on assisted suicide in Scotland is ambiguous. It is not specifically outlawed, but the issue has not yet been tested by the police or the courts.

The proposed legislation will require anyone who wants assisted suicide to sign a legal declaration while still mentally fit, a form of "living will". If the patient later opts for suicide, their mental state will then have to be assessed in a psychiatric examination to ensure they are not simply suffering from depression, MacDonald said.

When the draft bill is completed this autumn, it will be investigated by MSPs on the Holyrood health committee, and possibly the justice committee. These committees can amend the bill, before presenting it for a full parliamentary vote.

The British Medical Association, religious ethics groups and the major churches still vociferously oppose assisted suicide, and no Tory MSP has yet publicly supported the measure. Critics fear Scotland will become a centre for suicide tourism if no similar measures are passed in England.